This invention relates to a steerable drilling system for use in the formation of subterranean boreholes, for example for subsequent use in the extraction of oil or natural gas.
One form of steerable drilling system includes a bias unit having a plurality of bias pads. Each bias pad is moveable from a retracted position to an extended position by an associated piston operable by pressurised drilling fluid or mud, a suitable valve arrangement being provided to control the supply of drilling fluid or mud to each piston and thereby control the positions occupied by the bias pads at any given time.
The drilling fluid or mud sometimes carries with it particles or debris from further up the borehole being formed. The presence of such particles or debris can cause wear or malfunction of the valve arrangement and/or the pistons. Clearly this is undesirable. In order to reduce wear and the risk of malfunction it is known to provide a junk basket or filter to collect particles carried by the flow of drilling fluid or mud before the fluid or mud reaches the valve arrangement and pistons. Such devices are intended to remove fairly large particles which, potentially, could cause malfunction or blockage. They are not intended for, or suitable for, removal of small, dense particles which cause erosion.
It is an object of the invention to provide a steerable drilling system of reduced susceptibility to erosion.
According to the present invention there is provided a steerable drilling system comprising a bias unit, the bias unit having at least one actuator operable by pressurised drilling fluid or mud, in use, and a separator arranged to remove particulate matter from the supply of pressurised drilling fluid or mud to the or each actuator, the separator comprising a hydrocyclone.
The use of a hydrocyclone as the separator is advantageous in that it can be used to separate relatively dense particles from the supply of drilling fluid or mud, even when the dense particles are of fairly small dimensions and so may be capable of passing through a junk basket or filter. As it is the relatively small, dense particles that tend to cause erosion, the use of a hydrocyclone allows a reduction in erosion damage to be achieved.
The separator may be integrally formed with the bias unit. Alternatively, the separator and bias unit may be separate components, the separator being located upstream of, preferably immediately upstream of, the bias unit so that the drilling fluid or mud from which matter has been separated by the separator can be supplied directly to the bias unit.
The invention also relates to a hydrocyclone adapted for use in such a steerable drilling system.